The greater the magnification of a microscope objective, the smaller its focal length and the greater its refractive power. Consequently, correcting of the Petzval sum, a measure of the field (i.e., image plane) flatness, becomes difficult. In particular, since the refractive index differential across the most objectwise lens surface (i.e., the boundary surface or interface between the most objectwise lens and the immersion fluid) of an immersion objective is small, correction of the Petzval sum is difficult. Conventional microscope objective designs are such that correction of the Petzval sum and correction of chromatic aberration are mutually opposing so that simultaneous correction of both is difficult to achieve. In other words, if correcting the Petzval sum is emphasized, correcting chromatic aberration is difficult, and visa versa.
For example, increasing the lens diameter to increase the paraxial ray height h serves to correct the Petzval sum, since the radius of curvature of each lens surface can be increased. However, this approach is disadvantageous for correcting chromatic aberration, since the chromatic aberration coefficient is proportional to the square of paraxial ray height h. This results in residual chromatic aberration (e.g., secondary spectrum, and the like) which increases as the paraxial ray height h increases.
In addition, since the numerical aperture (NA) is generally large in a high-magnification microscope objective, spherochromatism and coma increase.
Thus, the ability to simultaneously correct the various aberrations, including chromatic aberrations and Petzval sum, is extremely difficult in present-day high-magnification immersion microscope objectives.
High-magnification immersion objectives are disclosed in Japanese Patent Application Kokai No. Hei 7-230039 and Japanese Patent Application Kokai No. Hei 7-281097. In the immersion objectives disclosed therein, a cemented lens having an embedded lens is arranged most objectwise. The Petzval sum is reduced and the field curvature is corrected by the radius of curvature and the refractive index differential across the cemented surface of the embedded lens.
On the other hand, in Japanese Patent Application Kokai No. Sho 58-192013, Japanese Patent Application Kokai No. Sho 61-275813 (Japanese Patent Application Kokoku No. Hei 5-67004) and in Working Example 1 of Japanese Patent Application Kokai No. Hei 5-142477, lens systems are disclosed that do not use an embedded lens to correct the Petzval sum.
The configurations disclosed in the abovementioned Japanese Patent Applications Kokai No. Hei 7-230039 and Kokai No. Hei 7-281097 and other like configurations are quite useful from the viewpoint of optical design. Particularly with apochromats, there are many cases of its use in objectives requiring a large NA. In actuality, however, there are practical difficulties from the viewpoint of actually fabricating the lens. In particular, the curvature of the concave surface on the embedded side of an embedded lens (i.e., the surface that contacts the embedded lens) becomes quite strong. Thus, not only does fabrication of this concave surface become difficult, but fabrication time and cost tend to increase. In addition, since the imagewise convex surface, which is the embedded side of the embedded lens, is often nearly hemispherical or beyond, it is difficult to polish this convex surface with high accuracy. The higher the magnification of the objective, the smaller its focal length, and the stronger the curvature of the abovementioned concave surface, and convex surface and the more difficult the lens is to fabricate.
On the other hand, in the lens systems disclosed the abovementioned Japanese Patent Applications Kokai No. Sho 58-192013, Kokai No. Sho 61-275813 (Japanese Patent Application Kokoku No. Hei 5-670004) and Kokai No. Hei 5-142477 (Working Example 1), balancing the correction of the Petzval sum with the correction of chromatic aberration and the like is inevitably sacrificed to the extent the Petzval sum is not corrected by the embedded lens.